This 2’ surface deflection of the vertical grid for Mexico, and North-Central is the
DMEX97 model. The computation used about one million terrestrial and marine gravity
measurements held in the NGS database as of March 1997. These gravity measurements
were augmented by data contributions from NGA (former National Imagery and Mapping
Agency), and satellitealtimeter derived gravity anomalies computed by Sandwell and
Smith (1997). The deflection values themselves were obtained by numericaldifferentiation
of cubic spline models along the parallels of the MEXICO97 geoid grid, followed by
a correction for curvature of the plumb line, yielding surface deflections of the
vertical. Since the EGM96 model was used to fill data gaps south of 20 degrees North
latitude, the deflections in and around those regions will be less accurate. The gravity
values are based on the International Gravity Standardization Net 1971 (IGSN71). The
deflections are referred to Geodetic Reference System 1980 (GRS80) ellipsoidalnormals
oriented in the International Terrestrial Reference Frame 1994 (ITRF94(1996.0)). Additional
information is available at:http://www.ngs.noaa.gov/We are particularly grateful to
NGA (former National Imagery and Mapping Agency) for their assistance and their data
contributions.